A method of silane-crosslinking a molded product of an ethylene copolymer graft-modified with an unsaturated silane compound, using water is described, for example, in Japanese Patent Publication No. 1711/13. This crosslinking method is advantageous over other methods, e.g., crosslinking using organic peroxides or radiation, in that equipment is simple, the limitations on the molding conditions are small, and the operation is easy.
For example, in the production of crosslinked electric power cables, the moldable temperature range in the crosslinking method using organic peroxides is from 20.degree. to 140.degree. C., whereas it is possible in the silane-crosslinking method to mold at the molding temperature of an uncrosslinked ethylene polymer, i.e., high temperatures.
Thus, the silane-crosslinking method which is industrially excellent is widely utilized in various fields, e.g., electric power cables, pipes, tubes, films, sheets, hollow moldings and foamed moldings.
An ethylene copolymer obtained by radical polymerization of ethylene and an unsaturated silane compound is economically advantageous and is used, as described, for example, in Japanese Patent Application (OPI) No. 9611/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
However, in molding and crosslinking by the silane-crosslinking using the ethylene copolymer, condensation reaction occurs at the initial stage in an extruder during extrusion molding and unevenness tends to occur on the surface of a molding. This is an extremely great defect which deteriorates the commercial value of the product, and the improvement thereof is strongly demanded.